On the brink. (Life/Environment).

Take a good look at these photos of the Iberian lynx and red-shanked Duoc langur monkey--because you'll probably never see these animals in the wild. Both species are critically endangered, meaning they're likely to go extinct within 10 years.

Sadly, their plight is only too common: 11,167 animal and plant species are threatened with extinction, according to the 2002 Red List, an index of threatened species released by the International Union for the Conservation of Nature (IUCN). "There's an extraordinary sense of loss when you see wonderful animals and plants vanish," says extinction expert Stuart Pimm of Columbia University in New York City.

Why do some species endure while others die off? "By far, the greatest reason is loss of habitat," says IUCN biologist Simon Stuart. The second major reason: invasire species, or non-native organisms (see "The Fish That Ate Everything," SW 12/13/02). Other threats include overhunting, disease, and climate change.

Usually a combination of factors contribute to the demise of a species. Consider, for example, the Iberian lynx. Its population plunged in the 1990s as its Mediterranean woodland habitat was flattened for agriculture and industry. The population further nosedived after humans introduced a virus to control the region's exploding wild-rabbit population. The virus worked, but the lynx lost its primary food source. Now, only 600 lynx remain in scattered pockets. "That makes it hard for individuals to find each other, and the rate of breeding decreases," says Stuart.

Species have come and gone throughout history, but conservationists estimate the current extinction rate is 1,000 to 10,000 times higher than it would be naturally. "Humans are responsible," Stuart says. "But we can find ways to conserve animal habitats."